Merge pull request #34 from mdonega/dev-20240528

added a comment on binder in the preface

book/preface.md

@@ -11,11 +11,10 @@ The material reported in this notes is largely a **compilation of existing liter
 
 So why yet another collection of notes ? What we tried to do is to improve the presentation of the material, sometimes using a simpler language, recurring to several practical examples, and by using the format of a "jupyter book".
 
-
 How to use these notes:
 - as an online book: just jump to the chapter/section (left and right columns) you are interested in, or browse it sequentially using the arrows at the bottom of the page. You also have a useful search tool at the top left of the webpage;
 - as a standard book: you can download the pdf of each chapter clicking the download icon;
-- as an interactive/executable book: download the ipynb of each chapter and execute it as a standard jupyter-notebook. If you are not familiar with notebooks you can find a quick howto in the appendices.
+- as an interactive/executable book: when you see the rocket icon on the top right of the page, you can click on it and run the section in binder. Otherwise you can download the corresponding ipynb and execute it as a standard jupyter-notebook. If you are not familiar with notebooks you can find a quick howto in the appendices.
 
 This notes are to be considered permanently as work in progress. Whenever something interesting comes our way, we will add it to the book.